RFR: 7903136: Don't require jdk18_home to be set in jextract gradle build [v4]

[Jorn Vernee]

On Tue, 29 Mar 2022 18:29:55 GMT, Jorn Vernee <jvernee at openjdk.org> wrote:

>> Drop the requirement to set `jdk18_home` in the gradle build, and rely instead on `JAVA_HOME` being set to JDK 18.
>> 
>> I've also updated the build instructions. The `-Pjtreg_home` flag seems to have been dropped by accident from the test command. I've re-added it. Also renamed `libclang_dir` -> `llvm_dir` which was missed in an earlier change.
>
> Jorn Vernee has updated the pull request incrementally with one additional commit since the last revision:
> 
>   Tabs to spaces

After some consideration and offline discussion. Implementing & maintaining our own JDK download code doesn't seem like it gives enough benefit for the cost.

For now we will stick with users manually having to supply a JDK 18 home. A local `gradle.properties` file could also be used to avoid having to pass this flag every time when invoking gradle. As well as using gradle with IntelliJ, which doesn't support passing command line flags to it's gradle builder (AFAICT).

Maybe once the toolchain feature supports jdk.java.net JDKs out-of-the-box, we could reconsider.

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PR: https://git.openjdk.java.net/jextract/pull/9